The present invention relates generally to thin film processing and in particular to a method and apparatus for depositing a thin film onto a substrate.
In recent years there has been great interest in the formation of metal doped polymer films. For example, it has been found that a thin-film coating having a desired color can be readily obtained by introducing metallic particulates into a plasma formed polymer.
Wielonski et al., U.S. Pat. No. 4,422,915 (hereinafter Wielonski), herein incorporated by reference, discloses a method of forming a colored polymeric film-like coating. The film-like coating is formed by depositing a plasma-formed polymer concurrently with particulates. In particular, Wielonski teaches that a plasma-polymerizable material is introduced into an electrical discharge region causing the plasma-formed polymer to be deposited. Concurrent with the formation of the plasma-formed polymer, the particulates are provided.
For providing the particulates, referring now to FIGS. 1, 2 and 3 of Wielonski, a filament resistance heater 29, an inductively heater evaporation source means 36, and an electron beam evaporator 40, respectively, are provided. (Filament resistance heater 29, source means 36 and electron beam evaporator 40 are hereinafter collectively referred to as evaporators.) However, these evaporators have several drawbacks.
One drawback is that each of the evaporators requires dedicated circuitry, e.g. power supplies, for powering the evaporators. Further, to couple the dedicated circuitry with the associated evaporator, one or more vacuum feedthroughs are necessary. Accordingly, the evaporators add complexity, decrease reliability and add cost to the apparatus.
Another drawback is that the evaporators waste the evaporated material and contaminate the process chamber. In particular, referring to Wielonski FIG. 1, evaporation material from filament heater element 29 has a tendency to not only to coat the substrate but also the rest of the apparatus. Thus some (if not most) of the evaporated material from filament heater element 29 ends up coating the inside of the apparatus instead of the substrate thus wasting the evaporation material. This is a particular disadvantage when the evaporation material is expensive, e.g. gold. Further, the buildup of evaporated material on the inside of the apparatus can flake thereby contaminating the apparatus including the substrate. The evaporators of FIGS. 2 and 3 of Wielonski suffer from the same drawbacks.
Accordingly, it is desirable to form a particulate containing polymeric coating using a method that does not require the use of dedicated circuitry to evaporate the particulate and does not waste evaporation material.
In accordance with the present invention, an apparatus for forming a thin film on a substrate includes a first gas inlet and an insert attached to the first gas inlet, the insert including a deposition source material such as lithium to be deposited on the substrate. The first gas inlet is coupled to a first process gas source which is typically a compressed gas cylinder of an inert gas such as argon.
The apparatus further includes a second gas inlet coupled to a second process gas source, the second process gas source including a first gas component source and a second gas component source. The first gas component source is typically a compressed gas cylinder of a reactive gas such as oxygen and the second gas component source is typically a container of organosilicon liquid such as hexamethyldisiloxane (HMDSO). Alternatively, the second process gas source is a container of organosilicon liquid and a reactive gas is not provided.
The apparatus further includes a conical shield attached to the first gas inlet and surrounding the end of the first gas inlet to which the insert is attached. The shield and first gas inlet are formed of an electrically conductive material and are electrically coupled to one another.
To form the thin film on the substrate, the substrate is mounted in a vacuum chamber formed of an electrically insulating material. After the vacuum chamber is pumped down to a subatmospheric pressure, the first process gas (e.g. argon) is provided through the insert which is shaped as an open ended hollow cylinder.
Power is then coupled to a main electrode adjacent an exterior surface of the vacuum chamber causing the first process gas within the insert to become ionized (i.e. to generate a plasma within the insert). Alternatively, instead of providing a main electrode adjacent an exterior surface of an electrically insulating vacuum chamber, the vacuum chamber is a grounded electrically conductive material and power is coupled to the first gas inlet and shield to ionize the first process gas inside of the insert. In either embodiment, the insert is heated causing the insert deposition source material (e.g. lithium) to vaporize from the insert forming deposition source material vapor (e.g. lithium vapor).
The deposition source material vapor mixes with the second process gas (e.g. oxygen and HMDSO or just HMDSO) provided from the second gas inlet in a shield plasma region defined by the shield. As a result, a PECVD thin film (e.g. silicon oxide) including the deposition source material (e.g. lithium) is deposited on the substrate.
Through the use of the shield, power coupling efficiency to the shield plasma region is higher than to the rest of the vacuum chamber volume resulting in a higher degree of ionization in the shield plasma region than in the rest of the vacuum chamber. Further, the shield concentrates the overall process gas mixture to the vicinity of the substrate. Accordingly, through the use of the shield, thin film deposition preferentially occurs on the substrate and not on the rest of the vacuum chamber avoiding waste of the insert material and the associated contamination of the vacuum chamber.
In one embodiment, an insert formed of an aluminum/lithium alloy is used to generate lithium vapor. Of importance, lithium vapor is generated without the use of pure lithium which is a chemically active and relatively hazardous material. Further lithium vapor is generated without having to provide an evaporator and the associated circuitry (e.g. power supplies) as in the prior art thus reducing the complexity, cost and increasing the reliability of the apparatus.
In another embodiment, a polymeric insert is used to introduce a polymer (e.g. polyethylene) into a PECVD formed thin film (e.g. silicon oxide). The structure is then heated to cause the polymer to vaporize and be removed from the PECVD formed thin film. The resulting thin film includes micro air gaps corresponding to the sites from which the polymer was vaporized. Accordingly, the resultant thin film has a low dielectric constant.
Advantageously, substantially any desired material can be incorporated into a PECVD formed thin film through the use of a suitable insert. Combined with the ability to readily change the PECVD formed thin film by varying the composition of the second process gas, any desired material can be incorporated into any desired PECVD formed thin film.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings.